This invention relates to an exhaust control valve for an engine and more particularly to an improved structure and method for precluding sticking of such exhaust control valves.
It has been proposed with two cycle engines to employ, in addition to the main exhaust port, an auxiliary exhaust port positioned above the main exhaust port and which has the flow through it controlled by an exhaust control valve. By opening this auxiliary exhaust port, the compression ratio is lowered and by closing the exhaust control valve, the compression ratio is increased. The same effect can also be achieved by providing an exhaust control valve in the main exhaust port which selectively closes and opens an upper portion of the main exhaust port. In this way, it is possible to operate the engine under a higher compression ratio under low load low speed conditions so as to improve the performance and reduce the compression ratio under high speed high load conditions to avoid excess pressures in the combustion chamber and pre-ignition or knocking. Hence, these types of exhaust control valves are extremely effective in improving the performance of an engine under particularly difficult running conditions without sacrificing performance under other running conditions. Furthermore, the exhaust control valve can be closed during starting to raise the compression ratio and facilitate starting.
However, there is a difficulty in conjunction with these valves. Since the exhaust control valve is subject to the exhaust gas temperatures, there must be some adequate clearance provided so as to permit the valve to rotate freely between its opened and closed position. Of course, this clearance cannot be so great as to cause leakage of pressure when the exhaust control valve is closed. Also, because of the different materials employed for the exhaust control valve then the engine casting in which it is rotatably journalled, compensation for different thermal expansions is necessary. Thus, the clearances actually will be less when the engine is cold than when it is warm. Therefore, on initial starting of the engine, it may be difficult to operate the exhaust control valve.
In addition, since these valves are normally employed in two cycle engines and since there may be oil present in the exhaust gases, this oil can cause a problem once the engine is shut off. When the engine is running, the temperature will be high enough to avoid any problem. However, as the engine cools, residual oil may carbonize or coke on the control valve and form deposits which along with other foreign matter will make it difficult to operate the valve when the engine is restarted. Of course, the temperature differences aforenoted also will aggravate this situation.
It is, therefore, a principal object of this invention to provide an improved exhaust control valve structure and method of operating an exhaust control valve for an engine.
It is a further object of this invention to provide an improved structure and method for cleaning an exhaust control valve under at least some conditions so as to avoid stickage on start up and when operating at low temperatures.
It is a further object of this invention to provide an improved, self-cleaning type of exhaust control valve for an engine.